U.S. patent number 6,755,499 [Application Number 10/113,856] was granted by the patent office on 2004-06-29 for printer device alignment method and apparatus.
This patent grant is currently assigned to Hewlett-Packard Development Company, L.P.. Invention is credited to Jorge Castano, David Toussaint.
United States Patent |
6,755,499 |
Castano , et al. |
June 29, 2004 |
Printer device alignment method and apparatus
Abstract
A method of determining a registration offset in a hard copy
apparatus, the apparatus comprising a pen arranged to mark a print
medium and a sensor arranged to detect marks on the medium along a
sensor path, the method comprising the steps of: marking a
alignment pattern on the medium, the pattern being at least
partially located along the sensor path; detecting the position
along the sensor path of a portion of the pattern; and, determining
a distance by which the pattern is offset from the sensor path in a
direction substantially perpendicular to the sensor path, the
pattern being configured such that the detected position is
indicative of the offset distance.
Inventors: |
Castano; Jorge (Barcelona,
ES), Toussaint; David (Barcelona, ES) |
Assignee: |
Hewlett-Packard Development
Company, L.P. (Houston, TX)
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Family
ID: |
26076523 |
Appl.
No.: |
10/113,856 |
Filed: |
March 28, 2002 |
Foreign Application Priority Data
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Mar 30, 2001 [EP] |
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01108128 |
Sep 4, 2001 [EP] |
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01121159 |
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Current U.S.
Class: |
347/19;
400/74 |
Current CPC
Class: |
B41J
2/2135 (20130101); B41J 29/393 (20130101) |
Current International
Class: |
B41J
2/21 (20060101); B41J 29/393 (20060101); B41J
029/393 (); B41J 007/18 () |
Field of
Search: |
;347/19,14,16,23,5,37,43,75,40,8-12 ;400/74 ;356/399-401
;250/548,568 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0744295 |
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Nov 1996 |
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EP |
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0863004 |
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Sep 1998 |
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EP |
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0867298 |
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Sep 1998 |
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EP |
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0895869 |
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Feb 1999 |
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EP |
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1027987 |
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Aug 2000 |
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EP |
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1033251 |
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Sep 2000 |
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EP |
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89/02826 |
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Apr 1989 |
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WO |
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Primary Examiner: Meier; Stephen D.
Assistant Examiner: Stewart, Jr.; Charles
Parent Case Text
This application is related to U.S. patent application Ser. No.
09/627,509, filed Jul. 28, 2000, entitled "Techniques for Measuring
the Position of Marks on Media and for Aligning Inkjet Devices",
which is hereby incorporated by reference and to U.S. application
Ser. No. 08/551,022, filed Oct. 31, 1995, entitled "Optical Path
Optimization for Light Transmission and Reflection in a
Carriage-Mounted Inkjet Printer Sensor", which is also hereby
incorporated by reference.
Additionally, this application is related to U.S. Pat. No.
5,835,108, entitled "Calibration Technique for Misdirected Inkjet
Printhead Nozzles", the disclosure of which is incorporated herein
by reference.
Claims
What is claimed is:
1. A method of determining a registration offset in a hard copy
apparatus, comprising the steps of: marking an alignment pattern on
a print medium with a first pen; traversing said pattern in a first
direction with a sensor and measuring the position of a portion of
said pattern in said first direction; and, determining the offset
of said pattern in a second direction, said pattern being
configured such that said measured position in said first direction
is indicative of a registration offset in said second
direction.
2. A method according to claim 1, wherein the step of determining
said pattern offset further comprises the step of referring to a
look up table relating values of said measured position to offset
distances or of carrying out a mathematical function on said
measured position value to determine said pattern offset.
3. A method according to claim 1, wherein said pen and said sensor
are each supported by a print carriage arranged traverse said
medium in positive and negative directions along a scan axis, said
scan axis being substantially parallel to said first direction.
4. A method according to claim 3, wherein said marking and
measuring steps are implemented during a movement of said carriage
in single direction along said scan axis.
5. A method according to claim 4, wherein said print medium is
maintained stationary relative to said apparatus between said steps
of marking and measuring said position of a portion of said pattern
in said first direction.
6. A method according to claim 1, wherein said pattern comprises a
plurality of points arranged to form a first line, said line lying
at an oblique angle relative to said first direction.
7. A method according to claim 6, wherein said pattern further
comprises a further plurality of points arranged to form a second
line, said second line being orientated at an angle substantially
perpendicular to said first direction and substantially separated
from said first line in said first direction.
8. A method according to claim 7, wherein said step of measuring
said position of a portion of said pattern comprises the step of
measuring the distance along a path followed by said sensor between
the points at which said first and said second lines are subtended
by said sensor path.
9. A method according to claim 1, wherein the apparatus further
comprises a further pen, wherein said method further comprises in
respect said further pen the steps of: marking a first further
alignment pattern on said print medium; traversing said first
further pattern in a first direction with said sensor and measuring
the position of a portion of said first further pattern in said
first direction; and, determining the offset of said first further
pattern in said second direction, said first further pattern being
configured such that said measured position in said first direction
is indicative of its registration offset in said second
direction.
10. A method according to claim 9, further comprising the step of
comparing said offset of the first pattern and said offset of the
first further pattern.
11. A method according to claim 10, further comprising the steps
of: marking with said further pen a second further alignment
pattern on said print medium spaced apart from said first further
pattern along said scan axis; repeating said measuring and
determining steps of claim 9 in respect of said second further
pattern, said second further pattern being configured to have a
measured position in said first direction indicative of a
registration offset in said second direction; and, comparing said
offsets determined in respect of said first and second further
patterns, to detect an error introduced into said registration
offset.
12. A method according to claim 11, further including the step of
determining the error in the measurement of said registration
offset of said alignment pattern printed by said first pen by
interpolating or extrapolating from said measured offsets of said
first and second further alignment patterns to the position along
said scan axis corresponding to the position of said alignment
pattern printed by said first pen.
13. A method according to claim 12, further including the steps of:
printing a further one or more alignment patterns with said first
pen extending substantially across said scan axis; repeating said
steps of determining said offset in said second direction and
determining said error in the measurement of said offset for each
of said one or more alignment patterns; and, determining an offset
correction based on the set of said offset errors of said one or
more alignment patterns.
14. A method according to claim 13, further including the steps of:
printing a further one or more further alignment patterns with said
further pen interspersed with said one or more alignment patterns
printed by said first pen; and using said one or more further
alignment patterns to establish the error in the offset measurement
of said further one or more alignment patterns printed by said
first pen.
15. A method according to claim 14, further including the steps of:
fitting a polynomial curve to three or more of said determined
offsets corresponding to the first, second or further alignment
patterns to increase the accuracy in determining said error in said
offset of said alignment pattern printed by said first pen by
interpolation or extrapolation.
16. A method according to claim 15, further comprising the step of
adjusting said print output position of either said first or said
further pen in dependence upon the relative offset of said first
and further pens including any detected error in the offset
measurement process.
17. A hard copy apparatus arranged to implement the method of claim
1.
18. A computer program comprising program code means for performing
the method steps of claim 1 when said program is run on a computer
and/or other processing means associated with suitable hard copy
apparatus.
19. A method according to claim 11, wherein said hard copy
apparatus is an inkjet apparatus, and said first and/or said
further pen comprises a plurality of ink ejection nozzles.
20. A method according to claim 19, wherein the step of adjusting
said print output position of at least one of said pens comprises
the step of adjusting the position of one of said pens in said
printer carriage or the step of excluding selected nozzles of the
printhead from use.
21. A method of determining a misalignment in a printer device,
said device comprising a pen arranged to mark a print medium and a
sensor arranged to detect marks on said medium along a sensor path,
said method comprising the steps of: marking an alignment pattern
on said medium, said pattern being at least partially located along
said sensor path and being configured such that the position along
said sensor path at which a predetermined portion of said pattern
is located is indicative of a distance by which said pattern is
offset from said sensor path in a direction substantially
perpendicular to said sensor path; and, detecting said position
along said sensor path of said predetermined portion.
Description
FIELD OF THE INVENTION
The present invention relates to printer devices, and particularly,
although not exclusively, to a method and apparatus for determining
and correcting misalignments between printheads in ink jet
devices.
BACKGROUND TO THE INVENTION
It is known to produce paper copies, also known as "hard" copies of
files stored on a host device, e.g. a computer using a printer
device. The print media onto which files may be printed includes
paper and clear acetates for use in lectures, seminars and the
like.
Referring to FIG. 1, there is illustrated a conventional host
device 1, in this case a personal computer, linked to a printer
device 2 via a cable 3. Amongst the known methods for printing text
or graphics and the like onto a print media such as paper it is
known to build up an image on the paper by spraying drops of ink
from a plurality of nozzles.
Referring to FIG. 2, there is illustrated schematically part of a
prior art printer device comprising an array of printer nozzles 4
arranged into parallel rows. The unit comprising the arrangement of
printer nozzles is known herein as a printhead. In a conventional
printer of the type described herein, the printhead 5 is
constrained to move in a direction 6 with respect to the print
media 7 e.g. a sheet of A4 paper. In addition, the print media 7 is
also constrained to move in a further direction 8. Preferably,
direction 6 is orthogonal to direction 8.
During a normal print operation, printhead 5 is moved into a first
position with respect to the print media 7 and a plurality of ink
drops 9a, 9b are sprayed from a number of printer nozzles 4
contained within printhead 5. This process is also known as a print
operation. After the completion of a print operation the printhead
5 is moved in a direction 6 to a second position and another print
operation is performed. In a like manner, the printhead 5 is
repeatedly moved in a direction 6 across the print media 7 and a
print operation performed after each such movement of the printhead
5. In practice, modern printers of this type are arranged to carry
out such print operations while the printhead is in motion, thus
obviating the need to move the printhead discrete distances between
print operations. When the printhead 5 reaches an edge of the print
media 7, the print media is moved a short distance in a direction
8, parallel to a main length of the print media 7, and further
print operations are performed. By repetition of this process, a
complete printed page may be produced in an incremental manner.
Since the advent of colour printing, printers with more than one
printhead are typically used. Generally, four printheads are used,
each storing and printing a different colour; for example: cyan;
magenta; yellow; and black. The inks from the four printheads are
mixed on the print media to obtain any other particular colour.
However, full colour printing requires that the inks from the
individual printheads are accurately applied to the print
media.
In order that this may be achieved, precise alignment of the
various printheads is required. The mechanical misalignment of a
printhead may result in an offset in the positioning of ink drops
on the print media. Such offsets may occur in the X direction (in
the media advance/media axis) or the Y direction (in the
carriage/scan axis). Additionally, angular offsets may also arise.
If each printhead in a printer is not sufficiently accurately
aligned with the remaining printheads of the printer, a
misregistration between the images formed by the different coloured
ink drops on the print media may result. This may cause too much
ink to be deposited in some areas and too little ink to be
deposited in others. This often gives rise "grainy" appearance in
the printed image. This type of print error is often particularly
noticeable to the viewer. Consequently, such misregistrations are
generally unacceptable, with colour printing typically requiring
image registration accuracy from each of the printheads of 1/2400
inch.
Various systems have been devised to address misregistration. In
particular, systems have been devised in order to ensure that
offsets in the X direction (media axis) are reduced to acceptable
levels. One such known system employs a unitary colour printhead,
which contains the nozzles of each ink colour: cyan; magenta; and
yellow. Thus, the nozzles of each ink colour may be accurately
aligned with those of the other colours on manufacture. Thus, when
the printhead is mounted in the print carriage of a printer, the
positions of the nozzles of each ink colour are constrained with
respect to each other. In this way, the operator need only ensure
that the colour printhead is correctly aligned with the black ink
printhead.
In this system, this is achieved by printing two overlying
alignment patches on the print medium, one with the black ink
printhead and the other with the colour printhead. Each alignment
patch consists of a series of parallel lines. However, the spacing
of the lines of the two alignment patches is slightly different,
thus giving rise to an interference pattern. When the alignment
patches have been printed, the operator manually inspects them to
determine the position in the overlying alignment patches of the
maximum or minimum ink density. From this information, the relative
offset between the two printheads in the media feed direction may
be determined.
Once this determination has been made, the processor of the printer
compensates for any offset in the media feed direction between
printheads by avoiding using those nozzles in each printhead that
extend in the media feed direction beyond the nozzles of the other
printhead. The processor of the printer also resets the "logical
zero" in terms of the nozzles' numbering in each printhead. That is
to say that the nozzles which are to be used in each printhead are
re-numbered, where necessary, such that the nozzles in each
printhead which correspond in terms of their position along the
media feed direction are allocated the same number, in order to
ensure correct registration between the images printed by the
different printheads. In this manner, the print output of the two
printheads may be aligned at the expense of a slightly reduced
number of usable nozzles.
This technique suffers from the disadvantage that it is relatively
slow, being non-automated and reliant upon an operator.
Furthermore, the process is less suitable for use in printers
having more than two printheads, due to the increased difficulty of
determining the relative offsets for a greater number of
printheads.
A second type of known system is generally used on large format ink
jet printers, which employ separate printheads for each ink colour.
In order to ensure that no misregistration occurs between the
images formed by the different coloured ink drops on the print
medium, an alignment routine is performed.
In this routine, alignment patches are printed across the sheet of
print media with each printhead so that they are approximately
aligned along the scan axis; i.e. in a direction perpendicular to
the media feed direction. The positions of the alignment patches in
the media feed direction are then measured using an optical
scanner, often referred to as a line scanner, which is mounted on
the printer carriage. This is achieved for each alignment patch by
positioning the line scanner at the appropriate point along the
scan axis so as to be able to detect the alignment patch and then
feeding the print media backwards (i.e. in a reverse feed
direction) so that the position of the patch on the media in the
media feed direction may be determined. The line scanner is then
positioned at the appropriate point along the scan axis to detect
the next alignment patch and the print media is fed forwards once
again in readiness for determining the position of the next patch
in the media feed direction. Once the position of each alignment
patch in the media feed direction has be determined in this manner,
the relative offsets in the media feed direction between the
individual printheads are calculated.
The print output of the different printheads are then aligned in
the media feed direction in the same manner as described above with
respect to the first type of prior art system; i.e. by avoiding
using those nozzles in each printhead that extend in the media feed
direction beyond the nozzles of the other printheads and by
resetting the "logical zero" in terms of the nozzles'
numbering.
Although this system functions satisfactorily, the process which it
employs is relatively slow, since the print media must be fed
backwards and then forwards again in order to measure the position
of each of the alignment patches. As the trend for increased
numbers of printheads in a printer continues, the duration of such
an alignment procedure is proportionally increased. Additionally,
this system suffers from a further problem in that it can only be
used with printer mechanisms that are capable of feeding the print
media in both a forwards and a reverse feed direction. Thus, this
technique is generally not applicable to printers in which the
reverse feed direction of the media feed motor is used to perform
other functions, such as powering a duplexing mechanism. Such
printers include many high production, small format printers.
It would therefore be desirable to provide a system and method for
determining a relative offset in the media advance direction
between the printheads of a printer, which overcomes one or more of
the disadvantages associated with the prior art.
SUMMARY OF THE INVENTION
According to a first aspect of the present invention there is
provided a method of determining a registration offset in a hard
copy apparatus, comprising the steps of: marking a alignment
pattern on a print medium with a first pen; traversing the pattern
in a first direction with a sensor and measuring the position of a
portion of the pattern in the first direction; and, determining the
offset of the pattern in a second direction, the pattern being
configured such that the measured position in the first direction
is indicative of a registration offset in a second direction.
By using an alignment pattern that is configured such that a
measurable distance associated with the pattern in a first
direction, for example along the scan axis of a printer device,
allows the placement of the pattern in a second direction, for
example along the media feed direction of the printer device, to be
determined several advantages are realised.
Firstly, the alignment pattern may be printed and then scanned in
the same direction, for example, along the scan axis direction of a
printer. Thus, the two processes may be implemented without having
to feed the print media, or having to scan the alignment pattern in
a direction different from that in which the alignment pattern was
printed. Thus, complex scanning arrangements may be avoided.
Moreover, this makes it possible to avoid the necessity associated
with some prior art methods of requiring the alignment patterns,
once printed, to be moved backwards and forwards under an optical
scanner in order to establish their position along the media feed
axis. As a consequence, the process by which the printheads offsets
in the media feed direction may be achieved according to the
present invention is comparatively rapid. This is because one pass
of an optical scanner across the print medium may be sufficient to
measure offsets of even a large number of printheads in the media
feed direction.
Preferably, the alignment pattern of the present invention
comprises two lines, one arranged parallel to the media feed axis
and a second arranged at 45 degrees to the first. By scanning a
narrow path across the scan axis of the media, intersecting both
lines, the distance between the two points in the scan path
intersected by the two lines may be measured. Due to the fact that
the two lines of the alignment pattern are arranged at 45 degrees
to each other, the measured distance will be equal to the
perpendicular distance from the scan path to the point at which the
two lines intersect. Thus, a change in the offset of a printhead in
the media feed axis will cause the position of the alignment
pattern, including both lines, to be offset relative to the scan
path. Therefore, the distance between the two points in the scan
path intersected by the two lines will change in proportion to the
offset. Thus, by measuring the distance between the point in each
line intersected by the scan path, the offset of the printhead in
the in the media feed axis may be determined.
Preferably, the method also includes the step of compensating the
measured registration offset for any errors introduced into the
measurement process by a non-constant pen-to-paper spacing in the
region of the alignment pattern. According to a preferred
embodiment of the present invention, this is achieved by
additionally printing two or more reference patterns, with a
further pen, in known positional relationships relative to the
alignment pattern. The reference patterns are printed with a single
printhead in order that no significant offset between the reference
patterns exists in the media feed direction. The reference patterns
are configured in a similar manner to the alignment pattern, in
that a measured position or distance in the first direction is
indicative of a registration offset in a second direction. By
determining what difference, if any, lies between the respective
positions of the reference patterns in the second direction, an
estimation of the error introduced into the measurement process by
a non-constant pen-to-paper spacing in the region of the reference
patterns may be obtained. The error in the position of the
alignment pattern may then be determined by interpolation.
Advantageously, this method also provides for a correction for any
errors introduced into the offset measurement process that might be
caused by skewing of the print media between the steps of printing
and scanning the alignment pattern. Thus, this embodiment makes the
invention highly suited to printer devices which have a scanner
located at a different point on the media path to the printheads;
for example downstream.
The present invention also extends to the corresponding apparatus
for implementing the above method. Furthermore, the present
invention also extends to a computer program, arranged to implement
the method of the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
For a better understanding of the invention and to show how the
same may be carried into effect, there will now be described by way
of example only, specific embodiments, methods and processes
according to the present invention with reference to the
accompanying drawings in which:
FIG. 1 illustrates a prior art printing system incorporating a
personal computer linked to a printer;
FIG. 2 illustrates schematically part of a prior art printhead in
relation to the print media on which it prints;
FIG. 3 shows a perspective view of a large format inkjet printer
incorporating the features of a first embodiment of the present
invention;
FIG. 4 shows a schematic perspective view of the carriage portion
of the printer of FIG. 3 showing a carriage-mounted optical
sensor;
FIG. 5 shows a schematic perspective view of the media positioning
system of the printer of FIG. 3;
FIG. 6 shows a view of the components of the optical sensor unit of
the printer of FIG. 3;
FIGS. 7a and 7b schematically illustrate the optical sensor of FIG.
6 located adjacent to a mark on a print medium, with FIG. 7a
illustrating the case in which the size of the mark is larger than
the field of view of the sensor and FIG. 7b illustrating the case
in which the size of the mark is smaller than the field of view of
the sensor;
FIG. 7c illustrates the spatial response of the sensor of FIG.
6;
FIG. 8a illustrates a schematic plan view of the printheads mounted
in the printer carriage assembly of the printer of FIG. 3 showing
the offset between printheads in the media feed direction;
FIG. 8b illustrates the schematic plan view of the printheads shown
in
FIG. 8a, showing the usable nozzles in each printhead once the
offsets between individual printheads in the media advance
direction have been determined using the method of the present
invention;
FIG. 9a illustrates printhead alignment patterns in accordance with
the first embodiment of the present invention;
FIG. 9b illustrates the path of the optical sensor as it passed
over the printhead alignment patterns of FIG. 9a;
FIG. 9c illustrates the changing output of the optical sensor as it
detects marks making up the printhead alignment patterns shown in
FIG. 9a and 9b;
FIG. 9d shows an enlarged view of a printhead alignment pattern
shown in FIG. 9a;
FIG. 10a illustrates printhead alignment patterns in accordance
with a second embodiment of the present invention and 10b
illustrates an enlarged schematic view of one of the printhead
alignment patterns shown in FIG. 10a; and,
FIGS. 11a-c each illustrate alternative printhead alignment
patterns in accordance with the present invention.
DETAILED DESCRIPTION OF THE BEST MODE FOR CARRYING OUT THE
INVENTION
There will now be described examples of the best mode contemplated
by the inventors for carrying out the invention.
First Embodiment
System of the First Embodiment
A typical application for the invention is in a large format colour
inkjet printer. Commonly assigned U.S. Pat. No. 5,835,108, entitled
"Calibration technique for misdirected inkjet printhead nozzles",
describes an exemplary system which can employ aspects of this
invention and the entire contents of which are incorporated herein
by reference.
Referring now to FIG. 3, the system of the present embodiment will
now be described. The figure shows a perspective view of an inkjet
printer 10 having a housing 12 mounted on a stand 14. The housing
has left and right drive mechanism enclosures 16 and 18. A control
panel 20 is mounted on the right enclosure 18. A print medium 33
such as paper is positioned along a vertical or media axis by a
media axis drive mechanism (shown in FIG. 5). As used herein, the
media axis is called the X-axis denoted as 13, and the scan axis is
called the Y-axis denoted as 15.
A carriage assembly 30, illustrated in phantom under a cover 22, is
adapted for reciprocal motion along a carriage bar 24 (i.e. along
the scan axis), which is also shown in phantom and is arranged to
support and position the four inkjet print cartridges 38, 40, 42,
and 44 (shown more clearly in FIG. 4) that store ink of different
colours, e.g., black, magenta, cyan and yellow ink, respectively.
The carriage assembly also holds the circuitry required for
interface to the ink firing circuits in the print cartridges. As
the carriage assembly translates relative to the medium 33 along
the X and Y-axes, selected nozzles in the inkjet print cartridges
are activated and ink is applied to the medium 33. The colours from
the three colour cartridges are mixed to obtain any other
particular colour.
The position of the carriage assembly 30 along the scan axis is
determined by a carriage positioning mechanism 31 with respect to
an encoder strip 32, as are illustrated in FIG. 4. FIG. 4 is a
perspective view of the carriage positioning mechanism 31 and the
encoder strip 32 together with the carriage assembly 30, which is
shown supporting the four print cartridges 38, 40, 42, and 44, and
positioned above the media roller 35b, of which a partial view is
shown. As can be seen from the figure, an optical sensor 50, which
is described below with respect to FIGS. 6 and 7, is connected to
the carriage assembly 30.
The carriage positioning mechanism 31 includes a carriage position
motor 31a which has a drive shaft and a drive roller 31b and 31c,
respectively, and which drives a belt 31d. The belt is secured by
idler 31e and is attached to the carriage 30. In this manner, the
position of the carriage assembly 30 may be moved in the Y-axis 15
along the carriage bar 24. The carriage assembly 30 may be moved in
either a positive or a negative direction, as is indicated by the
arrow 15 in the figure, in dependence upon the direction of
rotation of the motor 31a.
The position of the carriage assembly 30 in the scan axis is
determined precisely using the encoder strip 32. The encoder strip
32 is secured by a first stanchion 34a at one end and a second
stanchion 34b at the other end. An optical encoder strip reader
(not shown) is disposed on the carriage assembly 30 and provides
carriage position signals that are utilized to determine the
position of the carriage assembly 30 in the Y-axis 15.
FIG. 5 is a perspective view of a simplified representation of the
media positioning system 35 of the printer 10, in relation to the
printer carriage assembly 30. The media positioning system 35
includes a motor 35a, which is normal to and drives the media
roller 35b. The position of the media roller 35b is determined by a
media position encoder 35c on the motor. An optical reader 35d
senses the position of the encoder 35c and provides a plurality of
output pulses, which indirectly determine the position of the
roller 35b and, therefore, the position of the media 33 in the
X-axis.
The media and carriage position information is provided to a
processor on a circuit board 36 disposed on the carriage assembly
30 for use in connection with printhead alignment techniques of the
present invention.
FIG. 6 illustrates the optical sensor unit 50 of the printer 10.
The optical sensor 50 is arranged to sense marks or ink on the
print media 33, which have been ejected by the printheads 38, 40,
42, 44. As has been stated above, the optical sensor 50 is mounted
on the carriage assembly 30 and thus is free to sense marks on any
portion of the print media 33 by moving the printer carriage 30
and/or the media 33 to selected locations along the X and Y-axes,
respectively.
The specific sensor and method used in order to establish the
position of a line or mark on the print media does not form part of
the invention and any suitable, known sensor and method may be used
for this purpose. However, for the purposes of clarity, a suitable
optical sensor and method will now be briefly described. For a more
complete description of such an optical sensor and its method of
use, the reader is referred to U.S. patent application Ser. No.
09/627,509 filed Jul. 28, 2000, entitled "Techniques for measuring
the posit marks on media and for aligning inkjet devices", which is
assigned to the assignee of the present application, and is hereby
incorporated by reference. Additional details of the function of a
preferred optical sensor system and related printing system are
disclosed in U.S. application Ser. No. 08/551,022 filed Oct. 31,
1995 entitled "Optical path optimization for light transmission and
reflection in a carriage-mounted inkjet printer sensor", which is
assigned to the assignee of the present application, and is hereby
incorporated by reference.
FIG. 6 shows a more detailed view of the optical sensor unit 50
shown in FIG. 4. The optical sensor unit 50 includes: a photocell,
or optical detector 50a; a holder 50b; a cover 50c; an optical
element or lens 50d; and, a light source such as two LEDs 50e, 50f.
The optical sensor unit 50 in this exemplary embodiment includes
two LEDs, one green and one blue; the green LED being used to scan
all of the patterns or marks except the patterns or marks used to
obtain information from the yellow ink printhead.
A protective casing (shown in FIG. 4) that also acts as an ESD
shield for sensor components is provided for attachment to the
carriage. Also shown in the figure are the relative positions of
the object plane and the image plane that are offset from the plane
of the lens by distances S1 and S2, respectively.
The light from the light sources 50e, 50f illuminates the object,
such as a printhead alignment pattern printed on print media 33.
The image of the object is focussed by the optical element 50d on
the image plane and is detected by the optical detector 50a in a
conventional manner.
In operation, the optical sensor unit 50 is arranged to scan a
"line" across the print medium 33 in the scan or Y-axis direction
as the printer carriage assembly 30, to which the optical sensor
unit 50 is mounted, is moved across the scan axis. Where the
optical sensor unit 50 passes over areas of the print medium 33
with levels of reflectivity that differ from adjacent areas along
the scanned line, the signal output by the optical detector 50a
will vary in dependence upon the local changes in the detected
levels of reflectivity. Such areas include marks or portions of
alignment patterns printed on the print medium 33 by one of the
four inkjet print cartridges 38, 40, 42, and 44. In this manner,
changes in the output signal of the optical detector 50a can be
used to determine the position of a mark on the print medium
30.
This is illustrated in FIG. 7a. In the figure, the optical sensor
unit 50 is illustrated at the point that it passes over a mark 52a
as it traverses the scan axis (as indicated by the arrow in the
figure).
The optical detector 50a has a photosensitive area or areas which
produce electrical sensor signals 56a that follow the optical
transfer function (OTF) of the optical system. This OTF is the
response of the optical sensor to the light reflected from the
media. The spatial response of the sensor is the mapping of the
signal from the sensor in response to a point light source scanning
along the viewing area of the optical system. The optical response
can be defined mathematically as the "point spread function" (PSF),
i.e. the response of the detector system to light from a point in
space.
FIG. 7c illustrates the spatial response of the sensor, determined
by mapping the PSF along all the points of the space to be
analysed, here the space along the media plane. The values of the
coordinates in FIG. 7c for this example are in space coordinates of
1/1200 inch.
The sensor signal 56a output by the optical detector when the
sensor is scanning across the mark 52a on the media is the
mathematical convolution of the reflectivity of the mark 52a and
the spatial response of the optical sensor.
If the nominal size of the mark to be detected is similar to or
larger than the optical sensor viewing area, as indicated in FIG.
7a, the optical sensor signal is dominated by the shape of the
mark. Thus, the resulting sensor signal 56a has a plateau in the
maximum of the signal. The plateau adds inaccuracies in determining
the position of the centre of the mark. Furthermore,
non-uniformities in the marks on the medium can produce lack of
consistency of the plateau, introducing erroneous centre position
signals.
However, if the size of the mark to be detected is smaller than the
sensor viewing area, the sensor signal is dominated by the response
curve of the optical sensor. This is illustrated in FIG. 7b, where
the size of the mark 52b is smaller than the size of the viewing
area 54b of the sensor. This produces a corresponding sensor signal
56b, with a clear and relatively sharp peak. Therefore, in the
present embodiment, it is desirable that the marks or lines to be
detected are sized smaller than the sensor viewing area dimension
in the direction in which the measurement is to be made. In this
example, the application need only know the position along the scan
axis at which the centres of the marks are detected. Thus, the
dimension of the marks or lines can be made larger than the viewing
area in the media axis direction, but preferably are smaller than
the viewing area dimension in the scan axis direction.
Good results are typically obtained with a mark size between about
0.5 and 0.75 of the sensor viewing area dimension. Of course, the
smaller the mark in relation to the sensor viewing area, the higher
the resolution but at the expense of signal strength. In other
words, when the marks are made smaller than the viewing area of the
optical sensor, there is not a lower limit on the size of the mark,
and the designer is guided by the necessity of having a minimum
sensor signal to measure correctly. If the mark is very dark, a
smaller mark can be used, while obtaining better resolution. In
practice, the applicant has found that the measurement resolution
of this type of optical sensor may be up to 4 microns. This
provides a significantly greater resolution than the resolution or
nozzle spacing of an exemplary printer, which has a dot spacing of
1/1200 inches, which equates to a resolution of approximately 20
microns.
Thus, if the optical sensor can be modelled like a first order OTF
(corresponding to a normal curve), and the size of the mark is
smaller than the sensor viewing area, the position of the mark on
the media can be calculated with the precision of the mechanical
scanning system of the optical sensor. This system provides an
effective technique to find the centre of the mark because the
signal has a clear and sharp peak corresponding to the centre.
Referring now to FIG. 8a, a schematic plan view of the nozzle
plates of each of printheads 38, 40, 42 and 44 as mounted in the
printer carriage assembly 30 is shown. As can be seen from the
figure, each printhead has two columns of nozzles with a column
offset 41c. Furthermore, each printhead is separated from adjacent
printheads in the Y-axis or scan axis direction by a Y-axis offset
41a. Due to inaccuracies in the location of each printhead in the
printer carriage 30, each printhead is located slightly differently
along the X-axis or in the media feed direction, giving rise to
vertical printhead misalignments. By comparing the relative
positions along the X-axis of corresponding nozzles between two
printheads, while they remain on the carriage, it is possible to
determine an actual offset 41b between those printheads along the
media axis 13.
Method of the First Embodiment
The printhead alignment method of the present embodiment is
generally performed when a printhead is replaced, when the relative
offsets of one or more of the printheads in the media axis (X-axis)
are likely to change. This may be done either immediately on
replacing a printhead, or, when the printer is powered up and the
new printhead is detected. However, the method of the present
embodiment may also be manually triggered by a user using the user
interface 20 of the printer, at such a time as is determined by the
user. This may be done, for example, after a printhead crash has
occurred; i.e. when one or more printheads have come into contact
with the print medium and possible been moved relative to the
printer carriage assembly 30. Alternatively, the printer may be
programmed to implement the method of the present embodiment at
periodic intervals; for example, after a predetermined period of
time or after a predetermined amount of use.
When the method is implemented, the printer carriage assembly is
brought to the right hand end of the scan axis, as is shown in
FIGS. 3 and 4; i.e. adjacent the right hand drive mechanism
enclosure 18. The media positioning system 35 of the printer 10
then feeds the media 33 currently in the printer forwards, if
required, so that the method may be carried out using clean print
media.
The printer carriage assembly 30 is then controlled by the printer
control unit of the printer (not shown) to traverse the print media
33 along the scan axis 15 as in a normal printing mode. As the
printer carriage assembly 30 traverses the print media 33, each of
the four printheads, in sequence, prints an alignment pattern on
the print media 33 under the control of the printer control unit.
Each alignment pattern is printed using all of the nozzles in the
printhead. Thus, each alignment pattern has substantially the same
alignment characteristics as the printhead that printed it, whilst
it is mounted in the carriage assembly 30. Furthermore, the height
of each alignment pattern is therefore the same as the height of
the columns of nozzles of the printhead in the media movement
direction (X-axis); otherwise known as the "swath height" of the
printhead. Thus, any offset in the media axis of a given printhead
will be reflected in the position of the alignment pattern in the
media axis on the print medium.
FIG. 9a illustrates the four alignment patterns 61-64, which
respectively represent the black, cyan, magenta and yellow
alignment patterns printed by the printheads 61-64,
respectively.
As can be seen from the figure, in the present embodiment the
alignment patterns are identical, differing only in their placement
on the print medium 33. As can also be seen from the figure, each
alignment pattern consists of three straight lines 60a, 60b and 60c
(labeled only on alignment pattern 61 in the figure). Two of the
lines 60a and 60c are parallel to the media axis (X-axis) and are
positioned level with each other along the media axis. The third
line 60b joins one end of the line 60a and the opposing end of the
line 60c so as to form a line at 45 degrees to both the media axis
(X-axis) 13 and the scan axis (Y-axis) 15. For the purposes of the
present embodiment, the direction of the slope of the line 60c may
be varied. Thus, instead of sloping upwards from left to right as
is shown in the figure, the line 60b could instead slope downwards
from left to right in the figure.
Each of the alignment patterns is printed at a predetermined
location along the scan axis 15, as measured by the carriage
positioning mechanism 31 in conjunction with the processor on the
circuit board 36 of the carriage assembly 30. In this manner, it is
ensured that no two alignment patterns overlap. This means that it
is easier to distinguish one alignment pattern from another when
determining their positions on the print medium. However, the
skilled reader will appreciate that at least partially overlapping
alignment patterns may additionally or instead be used.
FIG. 9a also schematically illustrates that each of the alignment
patterns is positioned slightly differently along the media or
X-axis, due to the vertical misalignments of the printheads 38, 40,
42 and 44, as is illustrated in FIG. 8. As is the case in FIG. 8,
these misalignments have been exaggerated in FIG. 9 for the sake of
clarity.
Due to the relative positions in the printer carriage assembly 30
of the optical sensor unit 50 and the printheads 38, 40, 42 and 44,
the optical sensor unit 50 passes over the alignment patterns 61-64
shortly after they are printed; i.e. in the same pass of the
printer carriage assembly 30 over the print media 33 in which the
alignment patterns are printed. Thus, the skilled reader will
understand that in the present embodiment the print media 33
remains stationary between the step of printing the alignment
patterns and subsequently sensing the positions of the alignment
patterns with the optical sensor unit 50.
FIG. 9b illustrates the path 65 of the optical sensor unit 50
superimposed over the alignment patterns 61-64. The direction of
movement of the optical sensor unit 50 is shown by the arrows in
the figure.
As has been explained above with respect to the optical sensor unit
50, where the optical sensor unit 50 passes over printed marks, the
signal output by the optical detector 50a decreases in response to
the reduced levels of reflectivity of the printed marks relative to
the surrounding print medium 33.
FIG. 9c illustrates the signal 66 output by the optical detector
50a as it detects those portions of the alignment patterns 61-64
lying beneath the optical sensor unit path 65 shown in FIG. 9b. As
can be seen from FIG. 9c, the optical detector 50a outputs a narrow
pulse as it passes over each line 60a-c of each of the alignment
patterns 61-64. As has been explained above, the peak value of each
pulse corresponds to the detection of the centre of each
corresponding line.
Thus, for each alignment pattern 61-64 the optical detector 50a
outputs three detection pulses; A, B and C that correspond to the
detection of lines 60a, 60b and 60c, respectively. In FIG. 9c,
these detection pulses are labelled: A.sub.k, B.sub.k and C.sub.k
in respect of the black (k) alignment pattern 61; A.sub.c, B.sub.c
and C.sub.c in respect of the cyan (c) alignment pattern 62;
A.sub.m, B.sub.m and C.sub.m in respect of the magenta (m)
alignment pattern 63; and, A.sub.y, B.sub.y and C.sub.y in respect
of the yellow (y) alignment pattern 64.
As has been explained above with respect to FIG. 4, the
instantaneous position of the printer carriage assembly 30, as it
passes along the scan axis (Y-axis) is known. Consequently, the
position of the optical sensor unit 50, which is mounted with a
known offset to the printer carriage assembly 30, is also known at
the moment that the central, or peak value for each detection pulse
occurs, as is shown in FIG. 9c.
As the optical sensor unit 50 passes over each alignment pattern,
the printer control unit records the instantaneous positions of the
optical sensor unit 50 when the peak value of each of the detection
pulses A-C is output. These positions correspond to the positions
along the scan axis at which the three lines 60a-c are intersected
by the path 65 of the optical sensor unit 50.
In the case of each alignment pattern, the recorded position along
the scan axis of the optical sensor unit 50 at the moment that the
first line 60a is detected is subtracted from the position along
the scan axis of the optical sensor unit 50 at which the second
line 60b is detected. This yields the separation "d.sub.1 " between
the points at which the optical sensor unit path 65 crosses the
first and second lines 60a and 60b. This is shown in FIG. 9d, which
illustrates an enlarged view of the alignment pattern 61 together
with the overlying path 65 of the optical sensor unit as shown in
FIG. 9b.
Since the second line 60b lies at 45 degrees to the media movement
direction (X-axis), the separation "d.sub.1 " is also equal to the
distance "d.sub.2 " (also shown in FIG. 9d) between the point at
which the optical sensor unit path 65 crosses the line 60a and
furthest point of the line 60a in the direction of the negative
media feed direction (X-axis) as shown in the figure. Therefore,
the distance "d.sub.1 " indicates the length of the line 60a, and
indeed the alignment pattern 61 as a whole, which extends beyond
the optical sensor unit path 65 in the negative media feed
direction (negative X-axis). As has been stated above, the length
of the line 60a is known. In this embodiment, it is equal to the
swath height of the printhead that printed the alignment pattern
61. Therefore, the length of the line 60a, and thus the alignment
pattern 61 as a whole, which extends beyond the optical sensor unit
path 65 in the positive media feed direction (positive X-axis) is
given by:
The offset O.sub.b of the black alignment pattern 61 (i.e. the
distance by which the centre of the alignment pattern 61 is
displaced from the centre of the optical sensor unit path 65) in
the media feed direction (X-axis) relative to the optical sensor
unit path 65 may be given as an absolute distance by:
where a positive value offset indicates that the offset is in the
positive media direction (X-axis) and a negative value offset
indicates that the offset in the negative media direction
(X-axis).
The skilled reader will appreciate that the relative offset of the
alignment pattern may also be calculated, in the same manner as
described above, using the distance "d.sub.3 ", shown in the
figure, which separates the points at which the optical sensor unit
path 65 crosses the second and third lines 60b and 60c.
Due to the 45 degree relationship between the lines 60b and 60c,
the separation "d.sub.3 " is also equal to the distance "d.sub.4 "
(also shown in FIG. 9d) between the point at which the optical
sensor unit path 65 crosses the line 60c and furthest point of the
line 60c in the direction of the positive media feed direction
(X-axis) as shown in the figure.
Thus, using the same method described above using the measurement
"d.sub.1 ", the offset of the alignment pattern 61 in the media
feed direction (X-axis) relative to the optical sensor unit path 65
may also be given as an absolute distance by:
where similarly a positive value offset indicates that the offset
is in the positive media feed direction (X-axis) and a negative
value offset indicates that the offset in the negative media feed
direction (X-axis).
The skilled reader will appreciate that the offset in the media
feed direction (X-axis) for each alignment pattern may be measured
using either or both of the values "d.sub.1 " and "d.sub.3 ". By
using both values a check may be introduced into the procedure, in
that if the calculated offsets are not equal using both
measurements, then it may be concluded that an error has occurred
and that the routine should be performed again.
The offsets O.sub.c, O.sub.m and O.sub.y in the media feed
direction (X-axis) are then calculated in the same manner for the
cyan, magenta and yellow patterns 62-64, respectively.
Once this has been done, the relative offsets in the media feed
direction (X-axis) each of the printheads relative to one another
are calculated. In the present embodiment, this is achieved in the
following manner. The offset of each printhead O.sub.b, O.sub.c,
O.sub.m and O.sub.y is subtracted from the offset O.sub.b of the
black ink printhead 38. Thus;
Relative offset black=O.sub.b -O.sub.b =O
Relative offset cyan=O.sub.b -O.sub.c
Relative offset magenta=O.sub.b -O.sub.m
Relative offset yellow=O.sub.b -O.sub.y
Thus, the relative offsets for the cyan, magenta and yellow
patterns are determined relative to the black pattern, which is
deemed to have a zero relative offset. Once the relative offsets in
the media feed direction have been determined for each printhead,
this information is used by the printer control unit in order to
correct for any misalignment that there might be between the
printheads in the media feed direction. If there is a misalignment,
the print output of the different printheads are then aligned in
the media feed direction in the same manner as described above with
respect to the prior systems; i.e. by excluding from use nozzles in
each printhead that extend in the media feed direction beyond the
nozzles of the other printheads and by resetting the "logical zero"
in terms of the nozzles' numbering.
This is schematically illustrated in FIG. 8b, in which the minimum
value O.sub.min and the maximum value O.sub.max of the calculated
relative offsets are marked relative to the logical zero nozzle
Z.sub.1b of the black printhead 38. By "logical zero", it is meant
the nozzle of the black printhead in the most advanced point in the
X axis (positive direction as shown in the figure), which is
referenced by the number 0 in printing commands sent to the
printhead). The values O.sub.min and O.sub.max define between them
a band "A" across which not all of the printheads 38, 40, 42 and 44
have nozzles, as a result of their relative offsets in the X-axis.
The nozzles in each printhead that fall in this band are
accordingly not used in printing operations in order to ensure that
the print output of each printhead is correctly registered with
that of the remaining printheads in the X-axis.
As is shown in the figure, the black, cyan and yellow printheads
38, 40 and 44 have nozzles that fall into this band, including
their original logical zero nozzles: Z.sub.1b, Z.sub.1c and
Z.sub.1y, respectively. Thus, in the case of each of these
printheads a new logical zero nozzle is created which lies
approximately at the offset defined by O.sub.min These are
Z.sub.2b, Z.sub.2c and Z.sub.2y, respectively. The remaining
nozzles are then sequentially renumbered in a manner known in the
art. By contrast, the original logical zero nozzle Z.sub.1m lies on
the line O.sub.min. Thus, this nozzles of the printhead 42 are not
renumbered.
The same process of excluding nozzles from use is also applied to
the other end of the printheads. This may be done by creating an
exclusion band "B", of the same width as band "A" and extending
from the nozzle in the lowest position in the X-axis, labelled
N.sub.m of printhead 42, in the direction of the positive X-axis.
Thus, once the nozzles lying in band "B" have been excluded from
use, the number of working nozzles in each printhead is
substantially the same and arranged so that the swath position of
each printhead is coincident with the others, thus ensuring
improved print registration between the printheads.
Second Embodiment
The second embodiment generally fulfills the same functions as
described with respect to the first embodiment. However, the second
embodiment is arranged to compensate for certain position
measurement errors which might be incurred in the process of
scanning the printed test marks, due to the material properties and
positioning of the print media upon which the test marks are
printed.
An example of a phenomenon which may cause a position measurement
error to arise in the process of scanning the test patterns is
"cockle". Cockle is the term used to describe the wrinkling of the
print medium which has expanded due to absorbing liquid from the
ink. If the print medium in the region in which the test patterns
are printed is cockled, certain regions of the test patterns will
be located closer to the optical sensor unit 50 than would be the
case if the print media were to lie flat in the media plane; i.e.
the pen to paper spacing will vary across the test pattern. Due to
the relative orientations of the optical detector 50a and the light
sources 50e, 50f, this change in distance may cause an error in the
measurement of position of the test pattern along the path of the
optical sensor unit 65. A similar problem may arise in certain
printers in which the surface which supports the print media whilst
being printed on is not flat. For example, in some printers, this
surface is formed from a series of ribs arranged in the media feed
direction. Thus, in such printers, the ribs cause the print media
to lie in an undulating manner across the scan axis. This may cause
the same type of error in measuring the position of the test
patterns along the path of the optical sensor unit as if the print
media were cockled.
A further example of a phenomenon which may cause a position
measurement error to arise in the process of scanning the test
patterns is skewed print media, which may arise if the print media
is fed or otherwise moved in between the steps of printing the test
patterns and subsequently scanning the test patterns. Frequently,
the process of feeding print media in an incremental printer causes
the print media to move in a "snake-like" motion as it is skewed
repeatedly from side to side. The skewing of the test patterns
(i.e. rotating the test patterns slightly about the axis
perpendicular to the media plane) prior to being scanned,
introduces a direct error into the measurement of the relative
offsets between the printheads in the media feed direction. This
type of error may arise, in particular, in printers in which the
optical sensor is located away from (for example downstream) of the
printzone; thus necessitating a media feed operation between
printing and scanning the test patterns.
Therefore, the second embodiment is arranged to compensate for such
errors in order to ensure that the relative offsets between the
printheads in the media feed direction may be accurately measured
and then compensated for.
The second embodiment employs similar apparatus and methods to that
described with respect to the first embodiment, thus corresponding
apparatus and method steps will not be described further in
detail.
Referring to FIGS. 10a and 10b, the method of the second embodiment
will now be described. Features in FIGS. 10a and 10b which
correspond to features described in the first embodiment are
referenced with corresponding numerals.
As was described in the first embodiment, the printer carriage
assembly 30 is controlled by the printer control unit of the
printer to traverse the print media 33 along the scan axis 15 as in
a normal printing mode. As the printer carriage assembly 30
traverses the print media 33, three test patterns 70, 71 and 72 are
printed. These are shown in FIG. 10a. The first and third test
patterns 70 and 72 are printed by a single reference printhead; in
this example this is the black printhead 38. The second test
pattern 71 is printed by a different printhead, the offset in the
media feed direction of which is to be measured relative to the
reference printhead; in this example the measured printhead is the
cyan printhead 42. As can be seen from the figure, the second test
pattern 71 is printed between the reference test patterns 70 and 72
in the direction of the scan axis.
These test patterns each have the same form as those described with
reference to the first embodiment. Thus, the alignment patterns 70,
71 and 72 are identical, differing only in their placement on the
print medium 33. Further, they each consists of three straight
lines: lines 60a and 60c lying parallel to the media axis 13 and
being positioned level with each other along the media axis; the
line 60b joining one end of the line 60a and the opposing end of
the line 60c so as to form a line at 45 degrees to both the media
axis 13 and the scan axis 15. Again each test pattern 70, 71 and 72
is printed using all of the nozzles in the printhead and is printed
at a predetermined location along the scan axis 15. The relative
positions of the test patterns 70, 71 and 72 along the scan axis 15
are indicated by distances D1 and D2 in the figure.
As can be seen from the figure the test patterns 70 and 72 being
printed by the same printhead are printed level with each other in
the media axis 13. The test pattern 71, which is printed by a
different printhead is illustrated as having an offset in the media
feed direction relative to the other test patterns 70, 72. The
offset is illustrated in the figure by distance C.sub.0. The offset
C.sub.0 has been exaggerated in FIG. 10a for the purposes of
clarity.
Once the test patterns have been printed they are scanned in the
same manner as described in the first embodiment. However, this may
be done either in the same pass of the printer carriage over the
print medium as the printing of the test patterns, or in a
subsequent pass. Thus, the optical detector 50a outputs detection
pulses corresponding to detection of each of the lines 60a-c of
each of the three test patterns 70-72, which are used to determine
the positions of the test patterns in the media feed direction, as
is described below.
FIG. 10a illustrates the "apparent" path of the optical sensor unit
50 when it scans the test patterns 70-72, superimposed over the
test patterns 70-72. The "apparent" path of the optical sensor unit
50 is illustrated by the line L.sub.2. As can be seen from the
figure, the line L.sub.2 lies at an angle .alpha. to the direction
of the scan axis relative to the print media when the test patterns
70-72 were printed, which is represented by the line L.sub.1. FIG.
10a also illustrates the distance for each test pattern between its
vertex 60d (referenced only in the case of the test pattern 70) and
the point on its line 60a which is intersected by the line L.sub.2.
These distances are K.sub.1, C and K.sub.2 for test patterns 70, 71
and 72, respectively. The figure further illustrates the distances
between the points on lines 60a and 60b intersected by the line
L.sub.2, for each test pattern. These distances are A.sub.1, B and
A.sub.2, for test patterns 70, 71 and 72, respectively.
For the sake of convenience in demonstrating the calculation of the
offset distance C.sub.0, only, X and Y axes have been included in
the FIG. 10a. The X axis is parallel to the line L.sub.1 and
arranged such that the vertices 60d of both test patterns 70 and 72
lie on the X axis. The Y axis is positioned to be co-linear with
the line 60a of the test pattern 70.
FIG. 10a also illustrates the distance C.sub.1 between the X axis
in the figure and the point on line 60a of test pattern 71
intersected by the line L.sub.2.
As has been described above, there are various reasons why position
measurement errors might be incurred in the process of scanning the
printed test marks. In the case of a varying pen to paper spacing
across the scan axis 15, the skilled reader will appreciate that
the "actual" path of the optical sensor unit 50 may be parallel to
the direction of the scan axis relative to the print media when the
test patterns 70-72 were printed; i.e. the line L.sub.1. However,
the varying pen to paper spacing may introduce errors into the
measured distances lying between the different lines 60a-c of the
different test patterns 70-72; thus, giving the impression of a
deviation from the "actual" path of the optical sensor unit 50,
which corresponds to the "apparent" path L.sub.2.
The skilled reader will appreciate that the FIG. 10a represents
only a small proportion of the distance along the scan axis 15 of
the printer. Thus, in practice, where the pen to paper spacing
changes over the length of the scan axis, the angular divergence of
"apparent" path L.sub.2 of the optical sensor unit, relative to the
line Li will vary in dependence on the position along the scan
axis. Thus, in practice, the line L.sub.2 may trace a sinusoidal
path varying about the line L.sub.1 in the positive and negative
media feed axis 13 relative to the line L.sub.1.
However, where the print media on which the test patterns are
printed is skewed between being printed and scanned, the line
L.sub.2 may represent the actual path of the optical sensor unit 50
as it scans the test patterns 70-72. In this case, the angle
.alpha. represents the angle by which the print media is skewed,
for example, through a sheet feed operation.
The skilled reader will of course appreciate that in certain
circumstances both types of error may be simultaneously
present.
The processor of the printer determines the distances A.sub.1, B
and A.sub.2, separating the points at which the "apparent" path
L.sub.2 crosses the lines 60a and 60b in test patterns 70, 71 and
72, respectively. This may be achieved in the same manner that the
separation "d.sub.1 " was determined in the first embodiment.
The processor of the printer then determines the offset C.sub.0
between the cyan test pattern 71 and the black test patterns 70 and
72 in the media feed direction in the following manner.
The equation to the straight line represented by L.sub.2 may be
given by the equation:
The equation has boundary conditions: when X=O, Y=K.sub.1 ;
so, b=K.sub.1 ; and,
when.times.=D1+D2, Y=K.sub.2.
Therefore:
When X=D1, Y=C.sub.1, therefore: ##EQU2##
The offset distance C.sub.0 is equal to the C--C.sub.1, therefore:
##EQU3##
Referring to FIG. 10b, an enlarged trigonometric representation of
part of the test pattern 70 is shown. The figure illustrates the
lines L.sub.1 and L.sub.2, the angle .alpha.. As can be seen from
the figure, the distances and the directions of K.sub.1 and A.sub.1
are shown. Using trigonometry, the dotted line J1 is equal to:
Therefore: ##EQU4##
By analogy: ##EQU5##
In practice, it has been found that the size of the angle a is very
small.
Thus, as a tends to zero, K.sub.1 =A.sub.1, C=B and K.sub.2
=A.sub.2. The offset distance C.sub.0 is then given by:
##EQU6##
The skilled reader will however appreciate that the present
embodiment may also be applied to situations where the angle a is
not considered small. In such a situation, the required variables
may be calculated using conventional numerical methods. In the
present embodiment distance D1 is made equal to distance D2. Thus,
the offset distance C.sub.0 is given by: ##EQU7##
Thus, in the present embodiment of the invention, the offset
correction C.sub.O is determined by interpolating between the
measured distances for the two reference test patterns 70 and 72.
The offset distance C.sub.0 is then calculated by the processor of
the printer. The skilled reader will appreciate that in the case
where the print media has been skewed, but does is not cockled or
otherwise formed in order to cause a varying pen to paper spacing,
a single measurement of offset C.sub.0, may be sufficient to ensure
a good corrective adjustment; thus ensuring good alignment in the
media feed direction between the reference (black) printhead and
the cyan printhead. In this case, the processor of the printer then
implements the correction to the positioning of the cyan printhead
42 in the media feed direction. This may then be carried out in the
same manner as described in the first embodiment. The offset
distance relative to the black reference printhead is then
determined in the same manner for each of the remaining printheads;
thus ensuring that each printhead is satisfactorily aligned in the
media feed direction with the reference printhead.
However, in the case where a variation in the pen to paper spacing
is present across the scan axis, it will be appreciated it is
preferable to carry out a number of measurements of the offset
C.sub.0 at varying positions across the scan axis. Each of these
measurements may be carried out in the same manner as described
above. In this manner, an average value of the offset of the
printhead in question may be determined relative to the reference
printhead at varying positions across the scan axis. Thus, the
degree to which the offset is corrected may be selected such that
it gives good printing results across the whole length of the scan
axis along which the printing is carried out.
It will be understood that the greater the number of readings taken
across the scan axis, the better will be the correction to the
offset. However, the exact number of such measurements that need to
be carried out will depend upon the frequency and magnitude of the
pen to paper spacing variation as well as the required precision in
correcting the offsets in the media feed direction between the
printheads of the printer. These factors will vary depending upon
the situation in which the method of the present embodiment is
employed. However, this may be determined by experimentally.
In one preferred embodiment, a printhead under test prints a row
consisting of numerous test patterns across the scan axis, which
are alternated with test patterns printed by the reference
printhead. The skilled reader will understand that in this manner,
a given test pattern printed by the reference printhead may be
used, for interpolation purposes, to establish the relative offset
of test patterns printed on either side of it along the scan axis,
by the printhead under test printed.
The skilled reader will also appreciate that the present embodiment
need not be limited to calculating the relative offset of a given
test pattern by using a straight line interpolating technique
between two reference test patterns. Instead, for example, a
conventional curve fitting technique could be used to fit a
polynomial curve to the measurements of a number of reference test
patterns; i.e. greater than two. In this manner, the measured
offset of each test pattern printed by the printhead under test,
could be established relative to co-ordinates of the fitted curve
at the position along the scan axis corresponding to the position
of that test pattern.
The offset distance relative to the black reference printhead may
then be determined in the same manner for each of the remaining
printheads; thus ensuring that each printhead is satisfactorily
aligned in the media feed direction with the reference
printhead.
Further Embodiments
In the above embodiments numerous specific details are set forth in
order to provide a thorough understanding of the present invention.
It will be apparent however, to one skilled in the art, that the
present invention may be practiced without limitation to these
specific details. In other instances, well known methods and
structures have not been described in detail so as not to
unnecessarily obscure the present invention.
For example, the skilled reader will appreciate that the present
invention may be applied to devices other that ink jet printer such
as, for example traditional plotters which utilise felt-tipped pens
and the like. Similarly, although the above embodiment was
described with reference to colour printing, the skilled reader
will appreciate that the present invention is also applicable to
monochrome printers. Furthermore, although the above embodiment was
described with reference to a printer incorporating four
printheads, the skilled reader will appreciate the present
invention is also applicable printers that employ two, three or
more than four printheads. Indeed, the invention may also be used
to advantage with printers having only one printhead, should the
exact placement in the direction of the media axis of the printed
output need to be measured or controlled.
Additionally, the skilled reader will appreciate that the printhead
test patterns may be varied in a variety of ways. For example, it
will be clear to the skilled reader that the present invention may
be implemented using a reduced number of lines parallel to the
media axis (X-axis). For example, as is shown in FIG. 11a the
invention may be implemented using printhead alignment patterns
which have only one of lines 60a and 60c; in the case shown in the
figure, only line 60a.
Furthermore, the skilled reader will appreciate that assuming that
the position of printed output for each printhead is accurately
known, in the direction of the scan axis, then both of the lines
60a and 60c may be dispensed with in the printhead alignment
pattern. This is shown in FIG. 11b. In such an embodiment, the
position measurement normally made by measuring the position of the
line 60a along the scan axis may be replaced by the recorded
position along the scan axis of a nozzle that printed a particular
known point in the alignment pattern at the time that it was
printed; for example one or other of the ends a or b of the line
60b, as shown in FIG. 11b.
Additionally, although in the above embodiments each alignment
pattern was printed using all of the nozzles in the printhead, the
skilled person will appreciate that this need not be the case. For
example, each alignment pattern may instead by printed using just
selected nozzles of the printhead. For example half of the nozzles
in one column could be used, as is shown in FIG. 11c. In this
example, the nozzles located about the center of one column are
used in order to allow the patterns to be centrally located with
respect to the path of the optical sensor unit 50.
As can be seen from FIG. 11c this gives rise to smaller alignment
patterns, which use less print media in the media direction and
additionally used less ink. In such an embodiment, it is preferable
that generally corresponding nozzles are used by each printhead to
print the respective alignment patterns. In this manner, the
alignment patterns may each be arranged to overlap the path of the
optical scanner unit. Thus, the optical scanner may determine the
position of each of the alignment patterns in one pass of the print
media, without it being necessary to feed the print media in order
to individually position each alignment pattern in order that it
might be detected by the optical scanner.
Additionally, different alignment patterns may be used to implement
the present invention.
For example the angle of 45 degrees of the line 60b joining the two
lines 60a and 60c parallel to the media movement direction (X-axis)
may be varied to a different known angle. As the skilled reader
will appreciate, in the event that it is varied, there will no
longer be a unitary relationship between the printhead offset in
the media (X-axis) direction from the measurement made in the scan
axis direction. However, the printhead offset in the media
direction may in this case be determined by finding the measurement
made in the scan axis direction in a look up table relating
measurements made in the scan axis direction with printhead offset
in the media direction. Alternatively, a simple trigonometric
calculation may be preformed in order to determine the offset in
the media movement direction (X-axis) direction from the
measurement made in carriage movement direction (Y-axis).
A further example of a different alignment pattern which may be
used in conjunction with the present invention may include a curved
line or curved edge of a graphic instead of a straight line, such
as 60b of the above embodiments, for determining the printhead
offset in the media axis. In such an embodiment, provided the form
of the curve is known, the offset of the pattern in the media
direction may be determined from the measurement of the position of
the pattern in scan axis. Again, the printhead offset in the media
direction may be determined by finding the measurement made in the
scan axis direction in a look up table relating measurements made
in the scan axis direction with printhead offset in the media
direction.
Although all of the alignment patterns in the embodiments described
above were identical, the skilled reader will appreciate that this
need not be the case in practice. Thus, in further embodiments of
the invention, different alignment patterns may be used for
different printheads.
Furthermore, the skilled reader will realise that the present
invention may be implemented using a detector other than an optical
detector in order to determine the position of aspects of the
alignment patterns. Any suitable property of the mark which
differentiates it from the medium upon which it is located may be
used in order to determine its position. For example, if the
substance, for example ink, which is used to make the mark has
magnetic or conductive properties that may be used to differentiate
it from the background media, the invention may be implemented
using a sensor that detects the magnetic or conductive properties,
instead of the optical properties of the marks.
The skilled reader will also realise that in the case of the first
embodiment, the scanning step to detect the position of the
alignment patterns need not be performed on the same pass of the
carriage over the print media as that in which the alignment
patterns are printed. In practice this could be implemented on any
subsequent pass of the printer carriage over the print medium.
However, if the scanning step is implemented on the return pass of
the printer carriage or in any subsequent pass in the reverse
direction, the order in which the pulses output by the optical
detector as it passes over each line of each alignment pattern will
be reversed.
Although in the above embodiments the process of reducing the
offset in the media feed direction between printheads relies upon
excluding certain nozzles from use and resetting the "logical zero"
in terms of the nozzles' numbering, the skilled person will realise
that the other methods may be used to implement the present
invention. For example, once the relative offsets between the
various printheads have been measured, it would be possible to
correct these offsets using an electromechanical system to
physically move the printheads into alignment along the media
movement axis. This may be achieved for each printhead, for
example, by using a piezo-electric actuator to move the printhead
and a position sensor to detect the resultant change in position of
the printhead.
* * * * *